Preparation of polyimide from organic diisocyanate with alkali metal salt of alcohol as catalyst

ABSTRACT

The use of certain catalysts provide for an improved process for the preparation of soluble polyimides, polyamides, and polyamideimides. The catalysts are alkali metal salts of formula MOR, wherein R represents alkyl or aryl and M represents an alkali metal. The improved process comprises reacting organic diisocyanates with polycarboxylic compounds consisting of tetracarboxylic acids or the intramolecular dianhydrides thereof, tricarboxylic acids or the monoanhydrides thereof, dicarboxylic acids, and mixtures thereof, in the presence of said catalysts. The polymers are obtained in solution at low reaction temperatures and short reaction times thereby avoiding side-reactions which otherwise would be detrimental to polymer molecular weight and ultimate polymer properties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a novel process and is more particularlyconcerned with a novel process for the preparation of polyimides,polyamides, and copolymeric mixtures thereof.

2. Description of the Prior Art

The reaction of diisocyanates with dicarboxylic acids and dianhydridesin solution to form polyamides and polyimides is well known in thepolymer art; see for example U.S. Pat. No. 3,592,789 wherein there isdisclosed the formation of coating solutions by reacting a diisocyanate,such as 4,4'-methylenebis (phenylisocyanate), with trimellitic anhydridein dimethylformamide at about 150° to 300° F, and conversion to thecured polymer at 200° to 420° C. U.S. Pat. No. 3,541,038 discloses thepolymerization of trimellitic anhydride with diisocyanates at elevatedtemperatures; and U.S. Pat. No. 3,708,458 discloses the formation ofpolyimides requiring long reaction times. U.S. Pat. No. 3,701,756teaches the use of certain hydroxides and ureas as catalysts for thereaction of isocyanates with anhydrides, however such catalysts areextremely difficult to remove from the products so obtained. It is knownto those skilled in the polymer art that the reaction of diisocyanateswith dicarboxylic acids in solution to form polyamides requires evenhigher temperatures than those called for in the prior art hereinbeforecited; see for example U.S. Pat. No. 3,642,715.

It has been well established that isocyanates react with some commondipolar aprotic solvents such as dimethylformamide, dimethylacetamide,N-methylpyrrolidone, and the like, at elevated temperatures. See M. R.Weiner, J. Org. Chem. 25, 2245 (1960) and S. Terney et al., J. Polym.Sci., Part A-1,8, 683 (1970). For example, heating of phenylisocyanatein dimethylformamide at only 150° C for 150 minutes gives a 35% yield ofN-phenyl-N',N'-dimethylformamidine and 30% of a cycloaddition adductderived from a further reaction of the formamidine with four moles ofphenylisocyanate. The side reactions arising during polymerizationsinvolving the use of isocyanates in such solvents, have already beenconsidered; see The Reaction of Isocyanates with Polar Solvents, by H.Ulrich, paper presented at the University of Detroit, 1974 PolymerConference Series. The side reactions easily lead to chain termination(i.e., lowering of polymer molecular weight), or crosslinking, andincorporation of units other than amide or imide into the polymer chain,all of which are highly undesirable when high molecular weight, linearpolymers are desired.

I have now found a process for carrying out the polymerization reactionshereinbefore described and known from the prior art, said process beingfree of the difficulties described hereinabove. The novel process of thepresent invention provides for lower polymerization temperatures, andshorter polymerization times, when compared to the prior art. As anadded advantage to flow from the use of lower reaction temperatures,problems arising from possible solvent -- isocyanate interaction havebeen eliminated. Therefore the soluble polymers obtained by the processof the present invention are characterized by having excellent molecularweight.

SUMMARY OF THE INVENTION

This invention comprises a process for preparing an essentially linear,solvent soluble, polyimide, polyamide, or polyamideimide by thecondensation of an organic diisocyanate with the appropriatepolycarboxylic acid derivative in said solvent, the improvement whichcomprises carrying out said process in the presence of a catalyticamount of a compound MOR (I) wherein R represents alkyl or aryl, and Mis an alkali metal.

The term "alkali metal" means sodium, potassium, and lithium. The term"alkyl" means alkyl having from 1 to 18 carbon atoms, inclusive, such asmethyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl,decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl,heptadecyl, octadecyl, and isomeric forms thereof. A preferred range for"alkyl" is that wherein the carbon content is within the range of "loweralkyl" which means alkyl having from 1 to 8 carbon atoms inclusive. Theterm "aryl" means the radical obtained by removing one nuclear hydrogenatom from an aromatic hydrocarbon and is inclusive of phenyl, tolyl,xylyl, naphthyl, biphenylyl, and the like.

The term "solvent" means a dipolar aprotic solvent.

The term "appropriate polycarboxylic acid derivative" means adifunctional polycarboxylic compound containing two groups available toreact with the diisocyanate regardless of whether they be two carboxylicacid groups, two intramolecular carboxylic anhydride groups (or the freecarboxylic acids thereof), or one free carboxylic acid group with oneintramolecular anhydride group (or the free carboxylic acids thereof).

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention is applicable to the preparation of anypolyimide, polyamide, or polyamideimide which is soluble, at least tothe extent of about 5 percent by weight, in the reaction solvent used inits preparation. Such polyimides, polyamides, and polyamideimides are awell known class in the art, see for example: U.S. Pat. Nos. 3,063,966,3,541,038, 3,592,789, 3,642,715, 3,692,740, 3,696,077, 3,708,458,3,787,367.

The novel feature of the process of the invention lies in the use of theparticular catalyst set forth above. The procedure employed in carryingout the process of the invention is essentially that employed hithertoin the particular condensation with the notable exception that theaforesaid catalyst is always present in the reaction mixture.

The process of the invention is accomplished in the presence of acatalytic amount of at lest one compound of formula (I). By catalyticamount is meant an amount less than 1 mole per mole of isocyanateemployed. The amount of compound (I) employed is advantageously fromabout 0.001 mole to about 0.2 mole per mole of isocyanate, andpreferably is from about 0.002 mole to about 0.02 mole per mole ofisocyante. Compound (I) in excess of the proportions set forth can beemployed, if desired, but will afford no additional advantage.

The catalysts of formula (I) defined hereinbefore include the well knownalkali metal alkoxides, phenoxides, napthoxides, and the like. Themajority of said compounds are commercially available, or they can beeasily prepared from the appropriate alcohol and an alkali metal, suchas sodium, potassium, and lithium, in an inert solvent followed byremoval of the solvent (see, Experiments in Organic Chemistry, L. F.Fieser, p. 384, 1941, D. C. Heath and Co., New York, N. Y.) Typicalexamples of catalysts of formula (I) include: sodium methoxide,potassium methoxide, lithium methoxide, sodium ethoxide, potassiumethoxide, lithium ethoxide, sodium butoxide, potassium butoxide, lithiumbutoxide, sodium octoxide, potassium octoxide, lithium octoxide,potassium nonoxide, potassium tetradecoxide, potassium hexadecoxide,potassium heptadecoxide, potassium octadecoxide, sodium tert-butoxide,sodium isopropoxide, potassium tert-pentoxide, sodium phenoxide,potassium phenoxide, lithium phenoxide, sodium napthoxide, potassiumnapthoxide, lithium naphthoxide, and the like. A preferred group ofcatalysts of formula (I) consist of lithium methoxide, lithiumphenoxide, and sodium methoxide. A particularly preferred catalyst offormula (I) is sodium methoxide.

The process of the present invention is accomplished by bringingtogether in solution, under anhydrous conditions, a difunctionalpolycarboxylic compound, an organic diisocyanate and a catalytic amountof a compound of formula (I). It will be recognized by those skilled inthe art that reasonable precautions to exclude moisture should beexercised because of the tendency for isocyanates to react with water.Such precautions include the use of dry solvents, dry apparatus, andcarrying out the reaction under an inert atmosphere, i.e., nitrogen. Thereactants and conditions will be defined in detail hereinafter. In apreferred embodiment of the present invention the difunctionalpolycarboxylic compound and catalyst are dissolved in a dipolar aproticsolvent and the diisocyanate added thereto while the solution is beingheated and stirred. The stirring assists in achieving homogenity andadvantageously aids in the removal of the carbon dioxide formed duringthe polymerization reaction. While the procedure as set forth above is apreferred embodiment, it is to be understood that the process of thepresent invention can also be readily accomplished by the initialadmixture in solvent of all the ingredients which, upon heating, formthe corresponding polymers in solution. In a most preferred embodiment,the diisocyanate is added, as a solution dissolved in a dipolar aproticsolvent, to the heated solution comprising the polycarboxylic compoundand the catalyst of formula (I).

The process of the present invention is advantageously conducted atelevated temperatures from about 40° C to about 140° C and preferablyfrom about 60° C to about 130° C. Higher reaction temperatures can beemployed, however, such higher temperatures offer no advantage andinsofar as solvent -- isocyanate side reactions can occur thereat, theiruse is not particularly recommended.

The progress of the polymerization reaction is easily monitored by anysuitable analytical method known to one skilled in the polymer art. aparticularly suitable method is infrared analysis. The characteristicabsorptions arising from the isocyanate groups of the organicdiisocyanate (4.4μ), and the functional groups of the polycarboxyliccompounds such as the anhydride group (5.4μ), the carboxylic acid group(5.85μ), along with the characteristic absorptions of the polymersobtained therefrom such as the imide group (5.60, 5.80, and 7.25μ), andamide group (6.00μ), allow for the facile determination of the progressand completion of the polymerization. The reaction is continued untilthe diisocyanate and difunctional polycarboxylic compound are no longerdetectable by infrared absorption analysis.

The process of the present invention is advantageously accomplished in aperiod from about 2 hours to about 15 hours and preferably from about 4hours to about 10 hours.

Illustrative of the solvents used in the present invention aredimethylsulfoxide, diethylsulfoxide, dimethylformamide,diethylformamide, dimehylacetamide, diethylacetamide, tetramethylurea,hexamethylphosphoramide, N-methylpyrrolidone, tetramethylenesulfone, andmixtures thereof. A particularly preferred group of solvents consists ofdimethylformamide and N-methylpyrrolidone.

It will be appreciated by one skilled in the art that when mixtures ofdifunctional polycarboxylic compounds hereinafter described, are reactedwith a diisocyanate, the product is a random, or block copolymer,depending on the sequence of polycarboxylic compound addition.

The difunctional polycarboxylic compound employed in the process of theinvention contains at least two carboxylic moieties selected from theclass consisting of free carboxy groups, anhydride groups, and mixturesthereof. Said polycarboxylic compounds are inclusive of aromatic,aliphatic, cycloaliphatic or heterocyclic polycarboxylic acids as wellas the intramolecular anhydrides thereof, provided that, in the case ofthose anhydrides which contain a single anhydride group there is alsopresent in the molecule a free carboxy group. As will be appreciated byone skilled in the art only those polycarboxylic acids which containcarboxy groups attached either to two adjacent carbon atoms or to twocarbon atoms which are separated from each other by a single carbon orhetero-atom are capable of forming intramolecular acid anhydrides.

Any of the aforesaid polycarboxylic acids or anhydrides can be employedas the difunctional polycarboxylic compounds in the process of theinvention. As will be apparent to the skilled chemist the nature of therecurring units in the resulting polymers will vary according to thestructure of the starting difunctional polycarboxylic compound.

When the polycarboxylic compound is a dicarboxylic acid which isincapable of forming an intramolecular anhydride, the product formed inaccordance with the process of the invention is a polyamide e. g. theproduct from said dicarboxylic acid and a diisocyanate will contain therecurring unit ##STR1## wherein A is the hydrocarbon residue of thedicarboxylic acid starting material and B is the hydrocarbon residue ofthe diisocyanate. On the other hand, when the polycarboxylic compound isan intramolecular anhydride which contains two anhydride moieties orcontains one anhydride moiety and free carboxylic acid groups capable ofintramolecular anhydride formation, the product of reaction inaccordance with the process of the invention is a polyimide e. g. theproduct of reaction of a diisocyanate and a polycarboxylic compoundcontaining two intramolecular anhydride groups will contain therecurring unit ##STR2## wherein A' is the hydrocarbon residue of thedianhydride and B' is the hydrocarbon residue of the diisocyanate.

Similarly where the polycarboxylic compound contains one anhydride groupin addition to a free carboxylic acid group, the polymer resulting fromthe process of the invention will be a hybrid containing both amide andimide linkages.

All of the above types of polymers can be prepared in accordance withthe novel process hereinabove described and all fall within the scope ofthis invention. Thus, by appropriate choice of the polycarboxyliccompound it is possible to prepare any of a wide variety of polymersusing the single step process of the invention.

Illustrative examples of aromatic dicarboxylic acids employed in theprocess of the present invention include isophthalic acid andterephthalic acid. Illustrative example of aliphatic dicarboxylic acidsemployed in the present invention are malonic acid, succinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid,sebacic acid, 1-11-undecanedioic acid, 1,12-dodecanedioic acid andbrassylic acid. Illustrative examples of cycloaliphatic dicarboxylicacids include, 1,3-cyclopentanedicarboxylic acid, and1,4-cyclohexanedicarboxylic acid. a particularly preferred aromaticdiacid is isophthalic acid and, a particularly preferred aliphaticdiacid is brassylic acid.

Examples of polycarboxylic compounds which can be employed as the freecarboxylic acids or intramolecular anhydrides thereof, are:

trimellitic acid and the anhydride thereof,

pyromellitic acid and the dianhydride thereof,

mellophanic acid and the anhydride thereof,

benzene-1,2,3,4-tetracarboxylic acid and the dianhydride thereof,

benzene-1,2,3-tricarboxylic acid and the anhydride thereof,

diphenyl-3,3',4,4'-tetracarboxylic acid and the dianhydride thereof,

diphenyl-2,2',3,3'-tetracarboxylic acid and the dianhydride thereof,

naphthalene-2,3,6,7-tetracarboxylic acid and the dianhydride thereof,

naphthalene-1,2,4,5-tetracarboxylic acid and the dianhydride thereof,

naphthalene-1,4,5,8-tetracarboxylic acid and the dianhydride thereof,

decahydronaphthalene-1,4,5,8-tetracarboxylic acid and the dianhydridethereof,

4,8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-1,2,5,6-tetracarboxylicacid and the dianhydride thereof,

2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic acid and the dianhydridethereof,

2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic acid and the dianhydridethereof,

2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic acid and thedianhydride thereof,

phenanthrene-1,3,9,10-tetracarboxylic acid and the dianhydride thereof,

perylene-3,4,9,10-tetracarboxylic acid and the dianhydride thereof,

bis(2,3-dicarboxyphenyl)methane and the dianhydride thereof,

bis(3,4-dicarboxyphenyl)methane and the dianhydride thereof,

1,1-bis(2,3-dicarboxyphenyl)ethane and the dianhydride thereof,

1,1-bis(3,4-dicarboxyphenyl)ethane and the dianhydride thereof,

2,2-bis(2,3-dicarboxyphenyl)propane and the dianhydride thereof,

2,3-bis(3,4-dicarboxyphenyl)propane and the dianhydride thereof,

bis(3,4-dicarboxyphenyl)sulfone and the dianhydride thereof,

bis(3,4-dicarboxyphenyl)ether and the dianhydride thereof,

ethylene tetracarboxylic acid and the dianhydride thereof,

butane-1,2,3,4-tetracarboxylic acid and the dianhydride thereof,

cyclopentane-1,2,3,4-tetracarboxylic acid and the dianhydride thereof,

pyrrolidine-2,3,4,5-tetracarboxylic acid and the dianhydride thereof,

pyrazine-2,3,5,6-tetracarboxylic acid and the dianhydride thereof,

thiophen-2,3,4,5-tetracarboxylic acid and the dianhydride thereof,

and benzophenone-3,3',4,4'-tetracarboxylic acid and the dianhydridethereof.

Other anhydrides which may be employed in the practice of the inventionare; the intermolecular anhydride of trimellitic acid 1,2-anhydride(see, for example U.S. Pat. No. 3,155,687), the bis-anhydrides disclosedin U.S. Pat. No. 3,277,117 [e.g. 4,4'-ethylene glycol bis-anhydrotrimellitate and 4,4'-(2-acetyl-1,3-glycerol) bis-anhydro trimellitate]and the di-adducts of maleic acid or anhydride with styrene.

While any of the polycarboxylic acids and intramolecular anhydridesthereof defined and exemplified above can be employed in the preparationof the polymers of the invention, a preferred group of compounds forthis purpose are intramolecular anhydrides which are derived frompolycarboxylic acids having at least 3 carboxyl groups of which at leasttwo carboxyl groups are attached directly to an aromatic nucleus inortho-position with respect to each other. A preferred group ofpolycarboxylic acid intramolecular anhydrides are those selected fromthe class consisting of anhydrides having the following formulae##STR3## wherein R₁ represents a group selected from the classconsisting of carboxyl and the groups ##STR4## wherein the carbon atomsof the latter are each attached to adjacent carbon atoms in an aromaticring, and wherein X is a bridging group selected from the classconsisting of lower-alkylene, carbonyl, sulfonyl and oxygen. The term"lower-alkylene" means alkylene containing from 1 to 6 carbon atoms,inclusive, such as methylene, ethylene, 1,3-propylene, 1,4-butylene,2,3-butylene, 1,6hexylene and the like. A particularly preferred groupconsists of, benzophenone-3,3',4,4'-tetracarboxylicacid dianhydride,trimellitic anhydride, and mixtures thereof containing from about 10 toabout 90 mole percent of benzophenone-3,3',4,4'-tetracarboxylic aciddianhydride and from about 90 to about 10 mole percent of trimelliticanhydride.

It is to be understood that mixtures of the aforesaid intramolecularanhydrides with the dicarboxylic acid compounds hereinbefore set forthare within the scope of the present invention. A particularly preferredmixture consists of about 80 mole percent of trimellitic anhydride and20 mole percent of isophthalic acid.

The diisocyanates which can be employed in the process of the inventioninclude any of the known diisocyanates. Illustrative of thediioscyanates which are employed in the process of the invention are:2,4-toluene diisocyanate, 2,6-toluene diisocyanate,4,4'-methylenebis(phenylisocyanate), dianisidine diisocyanate, tolidinediisocyanate, 4,4'-diphenylether diisocyanate,4,4'-methylenebis(cyclohexylisocyanate), m-xylene diisocyanate,1,5-naphthalene diisocyanate, and the like. A preferred group ofdiisocyanates consists of, 2,4-toluenediisocyanate,2,6-toluenediisocyanate (and mixtures thereof),4,4'-methylenebis(phenylisocyanate) (MDI), and various mixtures of MDIwith the toluenediisocyanates. A preferred mixture consists of fromabout 10 to about 35 mole percent of 4,4'-methylenebis(phenylisocyanate)and from about 65 to about 90 mole percent of a member selected from thegroup consisting of 2,4-toluenediisocyanate, 2,6-toluenediisocyanate,and mixtures thereof.

A particularly preferred mixture consists of about 20 mole percent of4,4'-methylenebis(phenylisocyanate) and about 80 mole percent of amember selected from the group consisting of 2,4-toluenediisocyanate,2,6-toluenediisocyanate, and mixtures thereof.

The proportions of diisocyanate to difunctional polycarboxylic compoundemployed in the process of the present invention are from about 1.0 moleto about 1.10 mole per mole of polycarboxylic compound, and preferablyfrom about 1.0 mole to about 1.05 mole.

Upon completion of the polymeriztion reaction the polymer can be left insolution to be used thereafter without any further treatment. In analternative embodiment, the polymer is isolated in solid form bystandard methods known to those skilled in the polymer art. Inparticular, it is precipitated by pouring the polymer solution intorapidly stirred water, collection of the powdered polymer, followed bywashing with water and/or non-solvents, and finally drying to thefinished material. It will be recognized by those skilled in the artthat isolation of the polymer by precipitation in water willautomatically remove the trace amount of catalyst to be found therein.In an optional, and preferred step, the basic catalyst present in thefinal solution is neutralized by the addition of a minor amount of anacid, preferably a weak acid such as glacial acetic acid. Such aneutralization step obviates any difficulties that may be encounteredwhen the reaction solution of the polymer is to be used directly, e.g.in the making of films, fibers, or coatings.

The polymers prepared by the process of the invention can be employed inany of the uses to which high temperature resistant polyimides orpolyamides are currently put in the art. For example, the polymers ofthe invention in solid form can be molded in the form of bushings, sealfaces, electric insulators, compressor vanes and impellers, pistons andpiston rings, gears, thread guides, cams, brake lining, clutch faces,abrasive articles and the like. They can be employed in solution in thepreparation of coating compositions and can thereby be employed in wirecoating and in the casting or spraying of polymer films on a variety ofsubstrates such as metal, ceramic, fabrics, polymerics and the like.

Indeed, as the polymers prepared by the process of the invention formhigh molecular weight polymers soluble in organic solvents theyrepresent a particularly useful advance in the art since they provide ameans of molding or fabricating high temperature resistant polymers,including fibers, without the need to carry out a final chemicalreaction to produce the polymer in situ. The polymers also findparticular utility in the manufacture of articles having reinforcing ormodifying fillers and the like incorporated therein, including themaking of high temperature resistant laminates. Thus, fillers such asfiberglass, carbon fibers, graphite, molybdenum disulfide (to impartlubricity), powdered metals such as aluminum, copper and the like, andabrasive materials (for producing grinding wheels and the like) can beadded to solutions of the soluble copolyimides of the invention andintimately mixed therewith prior to removal of solvent followed by heatpressing or like techniques necessary to achieve production of thedesired article. Other processing advantages which accure from the hightemperature resistance, solvent solubility and thermoplasticity of thesecopolyimides of the invention will be apparent to one skilled in theart.

The following examples describe the manner and process of making andusing the invention and set forth the best mode contemplated by theinventors of carrying out the invention but are not to be construed aslimiting.

EXAMPLE 1

A dry 500 ml. resin flask equipped with a stirrer, condenser,thermometer, nitrogen inlet tube, and addition funnel was charged with64.4 g. (0.2 mole) of commercial grade (97.44% anhydride)3,3',4,4'-benzophenonetetracarboxylic acid dianhydride (BTDA) and 0.05g. (0.000915 mole of sodium methoxide catalyst. The flask contents weredissolved in 234 g. of dry dimethylformamide (distilled over calciumhydride). The temperature of the contents was raised to 80° C and,during constant stirring under nitrogen, a solution consisting of 10.0g. (0.04 mole) of 4,4'-methylenebis(phenylisocyanate) (MDI) and 28.0 g.(0.16 mole) of 2,4-toluenediisocyanate (TDI) dissolved in 30 g. of drydimethylformamide (DMF) was added dropwise over 4.5 hours. At the end ofthis time, infrared analysis of a sample of the viscous solutionrevealed only a trace amount of unreacted isocyanate (--NCO) andanhydride groups. An additional 2 mole percent excess [0.0008 mole (0.2g.) of MDI and 0.0032 mole (0.56 g.) of TDI ] of a mixture of thediisocyanates dissolved in 30 g. DMF was added over 1 hour.Approximately 6.5 hours from the beginning of the polymerization, IRanalysis revealed no unreacted --NCO or anhydride. The DMF solution,having an inherent viscosity, ηinh (0.5% at 29.1° C) = 0.41, consistingof approximately 25 percent by weight of copolyimide, was characterizedby a structure wherein approximately 80 percent of the recurringcopolyimide units had the formula ##STR5## and the remaining 20 percentof the recurring units had the formula ##STR6##

EXAMPLE 2

The following example is an uncatalyzed polymerization reaction that wasnot carried out in accordance with the present invention but is shownfor purposes of comparison.

Using the procedure and reactants set forth in Example 1 except for thefact that no catalyst was used, the polymerization described therein wasrepeated. At the reaction temperature of 80° C after 8.75 hours, IRanalysis showed an appreciable quantity of NCO and anhydride groupsremaining. Further, the solution which was 25 percent by weight insolids was quite turbid which was a result of the preferential reactionof the more reactive MDI to form the homo-polyimide which is known to beinsoluble, thereby leaving at least a portion of the TDI unreacted.

EXAMPLE 3

The following example is a polymerization reaction carried out in thepresence of a known catalyst for the reaction of an isocyanate with ananhydride (see J. Drapier, et. al., Tetrahedron Letters No. 6, 419-422,1973) but not a catalyst according to the present invention.

Using the procedure and reactants set forth in Example 1, except thatthe DMF was replaced by 175 g. of dry distilled N-methylpyrrolidone(NMP), the quantities of reactants were reduced by one half, and 0.05 g.(0.00015 mole) of dicobalt octacarbonyl was employed as the catalyst.After a 7 hour reaction period at 80° C, strong bands in the IRabsorption spectrum for --NCO and anhydride groups showed thepolymerization was proceeding only at a slow rate. As in Example 2, theturbidity of the polymerization solution was an indication of thepreferential formation of the insoluble MDI based polyimide. Thedicobalt octacarbonyl did not catalyze the copolymerization process.

EXAMPLES 4 - 7

Using the procedure and reactants of Example 1 and substituting thecatalysts set forth in Table I, the copolyimide according to Example 1was obtained in DMF solution in each of the examples.

                                      TABLE I                                     __________________________________________________________________________    Catalyst (wt. in g.)    Polymer Content (% by wt.)                            __________________________________________________________________________    Ex. 4 potassium phenoxide (0.12)                                                                      25                                                    Ex. 5 sodium phenoxide (0.10)                                                                         25                                                    Ex. 6 potassium methoxide (0.065)                                                                     25                                                    Ex. 7 potassium octoxide (0.17)                                                                       25                                                    __________________________________________________________________________

EXAMPLE 8

A dry one liter resin flask equipped as in Example 1 was charged with161 g. (0.5 mole) of purified BTDA, 0.2 g. (0.0037 mole) of sodiummethoxide catalyst, and 456 g. of dry DMF. The mixture was raised to 80°C under constant stirring and positive nitrogen flow to provide a clearsolution consisting of about 30 percent solids content. The additionfunnel was charged with a solution consisting of 25.0 g. (0.1 mole) ofMDI and 70.0 g. (0.4 mole) of a mixture consisting of 80 percent 2,4-TDIand 20 percent 2,6-TDI dissolved in 40 g. of DMF. The contents of theadditional funnel was added dropwise over a 6 hour period at 80° C. IRanalysis after 7 hours revealed only a trace of unreacted anhydride andno --NCO. An additional 0.25 g. (0.001 mole) of MDI and 0.70 g. (0.004mole) of the 80/20 mixture of 2,4- and 2,6-TDI (which is equivalent to a1 mole percent --NCO excess) was added over an additional 2 hour periodat 80° C. The final DMF copolyimide solution was characterized by havingan inherent viscosity, ηinh(0.5% at 29.6° C) = 0.665. There was thusobtained a copolyimide having the recurring unit ##STR7## in which 80percent of the recurring units R represented a mixture consisting of 80percent ##STR8## and 20 percent ##STR9## and in the remaining 20 percentof the recurring units R represented ##STR10##

A portion of the solution was precipitated into water in a WaringBlendor thereby obtaining the copolyimide as a finely divided powderwhich as thoroughly dried under vacuum at 140° C and 0.5 mm. The powderupon dissolution in DMF had an ηinh (0.5% at 29.6° C) = 0.47 and in NMPhad an ηinh (0.5% at 29.6° C) = 0.58. Films were cast from the DMFsolution of the copolyimide and then cured by either heating the filmbetween the platens of a press that were at 250° C without the use ofpressure, or else by curing in a vacuum oven at 180° C under about 1 mmpressure. Table II sets forth a comparison of the physical properties ofthe films cured by both of these methods.

                  TABLE II                                                        ______________________________________                                        Comparison of Press and Vacuum Cured Film                                     ______________________________________                                        Press cured:   ∥ to film length                                                                 ⊥ to film length                              ______________________________________                                        Average thickness (mils)                                                                     3.1         2.2                                                Tensile Str. (psi)                                                                            16,480      18,630                                            Tensile Modulus (psi)                                                                        400,060     312,720                                            Elongation (%) 4.7         3.4                                                Vacuum cured:                                                                 Average thickness (mils)                                                                     2.1                                                            Tensile Str. (psi)                                                                            17,110      19,800                                            Tensile Modulus (psi)                                                                        469,200     417,600                                            Elongation (%) 5.1         7.2                                                ______________________________________                                    

EXAMPLE 9

A dry 500 ml. resin flask equipped with a thermometer, stirrer, nitrogeninlet tube, condenser, and addition funnel was charged with 32.2 g. (0.1mole) of purified BTDA and 19.2 g. (0.1 mole) of sublimed trimelliticanhydride (TMA) dissolved in 200 g. of dry NMP along with 0.05 g.(0.0005 mole) of lithium phenoxide catalyst. During stirring and under aslight positive pressure of nitrogen the solution temperature was raisedto 105° C and 17.4 g. (0.1 mole) of 2,4-toluenediisocyanate was addeddropwise over a 1 hour period. Then, 25.0 g. (0.1 mole) of MDI dissolvedin 25 g. of NMP was added dropwise over 6.5 hours at 105° C. At the endof the addition period the clear viscous solution as diluted with 77 g.of NMP thereby reducing the solids content to 20 percent by weight. Thecatalyst was neutralized by the addition of about 0.1 g. of glacialacetic acid to the solution. The NMP polymer solution was characterizedby having an ηinh (0.5% at 29.6° C) = 0.91. There was thus obtained ablock copolymer with respect to the isocyanate residue whereinapproximately 50 percent of the recurring units had the structure##STR11## and the remaining 50 percent had the structure ##STR12## andfurther, wherein 50 percent of the recurring units were those in which Rrepresented ##STR13## and the remainder were those in which Rrepresented ##STR14##

Films were easily cast from the NMP solution and possessed the followingproperties after vacuum curing in accordance with the conditionsdescribed in Example 8

    ______________________________________                                        Tensile Str. (psi)    15,960                                                  Tensile Modulus (psi)                                                                              465,100                                                  Elongation (%)       7.4                                                      ______________________________________                                    

EXAMPLE 10

A dry 500 ml. flask equipped as described in Example 1 was charged with38.4 g. (0.2 mole) of sublimed TMA and 0.08 g. (0.0015 mole) of sodiummethoxide catalyst along with 244 g. of NMP (dried by distillation fromcalcium hydride). The temperature of the solution was raised to 115° Cwhile the solution was stirred under nitrogen. The addition funnel wascharged with 51.0 g. (0.204 mole, a 2 mole percent excess) of MDIdissolved in 40 g. of NMP and the isocyanate solution was slowly addedover 5 hours. A further quantity of MDI, 1.0 g. (a further 2 molepercent excess) dissolved in 4 g. of NMP was added over a 2 hour period.The catalyst was neutralized by the addition of 0.2 l g. of glacialacetic acid. The polymer solution was characterized by having an ηinh(0.5% at 30° C) = 0.69. There was thus obtained a polyamideimide havingthe recurring unit ##STR15##

Films were cast from the NMP solution, and cured as previously describedand the resulting films having an average thickness of 3 mils possessedthe following properties

    ______________________________________                                                         ∥ to length                                                                    ⊥ to length                                   Tensile Str. (psi)                                                                              13,910    13,720                                            Tensile Mod. (psi)                                                                             397,100   363,900                                            Elongation (%)   8.8       7.7                                                ______________________________________                                    

EXAMPLE 11

A dry 500 ml. resin flask equipped as in Example 1 was charged with30.75 g. (0.16 mole) of sublimed trimellitic anhydride, 6.65 q. (0.04mole) of purified isophthalic acid, and 0.08 g. (0.0015 mole) of sodiummethoxide catalyst. The contents were dissolved in 161 g. of dry NMP andthe stirred solution heated to 120° C under nitrogen. A solution of 51.0g. (0.204 mole) of MDI dissolved in 50 g. of NMP was added to thestirred solution at 120° C over a 4 hour period. At the end of thisperiod an IR spectrum had no absorption bands for unreacted --NCO oranhydride. A solution of 0.2 g. of glacial acetic acid dissolved in 69g. NMP was added to the solution as it was cooling down resulting inpolymer solution having about 20 percent solids content andcharacterized by having an η(0.5% at 30.4° C) = 0.74. There was thusobtained a copolyamideimide wherein 80 percent of the recurring unitshad the structure ##STR16## and the remaining 20 percent had thestructure ##STR17##

Films were cast and cured as previously described and possessed thefollowing properties in the parallel and perpendicular direction to thefilm length.

    ______________________________________                                                         ∥ to length                                                                    ⊥ to length                                   Tensile Str. (psi)                                                                              13,600    15,050                                            Tensile Mod. (psi)                                                                             406,500   365,700                                            Elongation (%)   11.0      17.5                                               ______________________________________                                    

EXAMPLE 12

A dry 500 ml. resin flask equipped according to Example 1 was chargedwith 49.8 g. (0.3 mole) of purified isophthalic acid and 0.1 g. (0.0026mole) of lithium methoxide. The flask contents were dissolved in 240 g.of dry NMP by stirring under nitrogen and the solution heated to 115° C.A solution consisting of 45.0 g. (0.18 mole) of MDI and 20.88 g. (0.12mole) of a mixture of 80 percent 2,4-TDI and 20 percent 2,6-TDIdissolved in 28 g. of NMP was added to the flask at 115° C over a 6 hourperiod. An additional 0.84 g. (0.0048 mole) of the 2,4-and 2,6-TDImixture along with 1.75 g. (0.0072 mole) of MDI were diluted with about10 g. of NMP and added to the flask over a 2 hour period. The solutionbecame very viscous and a solution of 0.2 g. of glacial acetic aciddissolved in 92 g. of NMP was added to reduce the solids content toabout 20 percent. The polymer solution was characterized by having anηinh (0.5% at 30° C) = 0.54. There was thus obtained a copolyamidehaving the recurring unit ##STR18## in which in 60 percent of therecurring units, R represented and, in the remaining 40 percent, Rrepresented a mixture consisting of 80 percent ##STR19## and 20 percent##STR20##

EXAMPLE 13

The following example is a polymerization reaction that was not carriedout in accordance with the present invention.

Using the same procedure and reactants set forth in Example 12, thepolymerization was repeated, however the catalyst was replaced by 0.126g. (0.0006 mole) of the disodium salt of isophthalic acid. The samereaction conditions and excess of isocyanate as used in the previousexample were employed in the present example. However, the films castfrom the polymer solution were brittle and therefore the copolyamide soobtained was adjudged to be not of sufficiently high enough molecularweight to be useful.

EXAMPLE 14

Using the same procedure and apparatus employed in Example 12, thepolymerization was repeated with the following exceptions. The purifiedisophthalic acid used was 53.15 g. (0.32 mole) along with 60.0 g. (0.24mole) of MDI, 13.92 g. (0.08 mole) of 2,4-TDI and 0.08 g. (0.0015 mole)of sodium methoxide. The reaction time was 7 hours at 120° C followed byan additional 5 hours during which time a 5 mole percent excess of MDIwas added. The resulting polymer solution was very viscous. There wasthus obtained a copolyamide having the recurring unit as set forth inExample 12 in which, in 75 percent of the recurring units, R represented##STR21## and, in the remaining 25 percent, R represented ##STR22##

Films were cast from the solution and cured either by heating the filmbetween the platens of a press that were at 250° C without the use ofpressure, or else by curing in a vacuum oven at 180° C under 1 mmpressure. The inherent viscosity of a sample of film dissolved in conc.H₂ SO₄ was ηinh (0.1% at 30.3° C)= 0.790.

Table III sets forth a comparison of the film properties prepared bothways.

                  TABLE III                                                       ______________________________________                                        Comparison of Press and Vacuum Cured Film                                     ______________________________________                                        Press cured:   ∥ to film length                                                                 ⊥ to film length                              ______________________________________                                        Average thickness (mils)                                                                     2.8                                                            Tensile Str. (psi)                                                                           13,040      13,420                                             Tensile Modulus (psi)                                                                        459,800     413,200                                            Elongation (%) 5.8         6.7                                                Vacuum cured:                                                                 Average thickness (mils)                                                                     3.1                                                            Tensile Str. (psi)                                                                           13,850      13,070                                             Tensile Modulus (psi)                                                                        445,500     380,900                                            Elongation (%) 7.1         9.2                                                ______________________________________                                    

EXAMPLE 15

A dry 500 ml. resin flask equipped as in Example 1 was charged with48.82 g. (0.2 mole) of purified brassylic acid, and 0.08 g. (0.0015mole) of sodium methoxide dissolved in 200 g. of dry NMP. Thetemperature was raised to 115° C and 50.0 g. (0.2 mole) of MDI dissolvedin 44 g. of NMP was added dropwise over a 4 hour period. A 1 molepercent (0.5 g.) excess of MDI dissolved in 14 g. of NMP was added over1 hour. Twelve drops of glacial acetic acid were added to the paleyellow solution to neutralize the catalyst. The polymer solutioncontained 24 percent by weight solids. The polymer was precipitated intowater in a Waring Blendor, collected, washed with acetone, and finallydried by heating at 145° C in vacuum overnight. The polyamide wascharacterized by having an inherent viscosity in m-cresol of ηinh (0.5%at 30° C)= 0.69. There was thus obtained a polyamide having therecurring unit

I claim:
 1. In a process for preparing an essentially linear dipolaraprotic solvent soluble solid polyimide by the condensation of anorganic diisocyanate with a polycarboxylic compound containing twointramolecular carboxylic anhydride groups or free carboxylic acidsthereof under anhydrous conditions in said solvent, the improvementwhich comprises carrying out said process in the presence of a catalyticamount of a compound MOR, wherein R represents alkyl or aryl, and M isan alkali metal and at a temperature of from about 40° C to about 140°C.
 2. The process according to claim 1 wherein the catalyst is lithiumphenoxide.
 3. The process according to claim 1 wherein the catalyst islithium methoxide.
 4. The process according to claim 1 wherein thecatalyst is sodium methoxide.
 5. The process according to claim 1wherein the polycarboxylic acid derivative comprises an aromatictetracarboxylic acid dianhydride.
 6. A process according to claim 1wherein said solid polyimide remains dissolved in said dipolar aproticsolvent.
 7. In a process for preparing an essentially linear dipolaraprotic solvent soluble solid polyimide by the condensation of anorganic diisocyanate with a polycarboxylic compound containing twointramolecular carboxylic anhydride groups or free carboxylic acidsthereof under anhydrous conditions in said solvent, the improvementwhich comprises carrying out said process in the presence of a catalyticamount of a compound MOR, wherein R represents alkyl, and M is an alkalimetal and at a temperature of from about 40° C to about 140° C.
 8. Theprocess according to claim 7 wherein the polycarboxylic acid derivativeis 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride.
 9. Theprocess according to claim 8 wherein the diisocyanate is a mixturecomprising from about 10 to about 35 mole percent 4,4'-methylenebis(phenylisocyante) and about 65 to about 90 mole percent of a memberselected from the group consisting of 2,4-toluenediisocyanate,2,6-toluenediisocyanate, and mixtures thereof.
 10. The process accordingto claim 9 wherein the diisocyanate is a mixture comprising about 20mole percent of 4,4'-methylenebis(phenylisocyanate) and about 80 molepercent of a member selected from the group consisting of2,4-toluenediisocyanate, 2,6-toluenediisocyanate, and mixtures thereof.11. The process according to claim 11 wherein the dipolar aproticsolvent is dimethylformamide.
 12. The process according to claim 11wherein the catalyst is sodium methoxide.
 13. The process according toclaim 10 wherein the dipolar aprotic solvent is N-methylpyrrolidone.